According to my knowledge, open source software is free to use and distribute. If modified it should be distributed under the same license. But what about only using output of the programme?
Eg. Consider sharelatex and overleaf. They are well known online latex editors. While compiling latex code they use
pdflatex program, which is open source. Now, as they are web-based they are not redistributing it, but they are using output of the program. It is fine till here, but when it comes to revenue. Do they have to pay royalties?
It will be great if someone can concentrate on lppl at http://www.latex-project.org/lppl/.